In recent years, the demands for ink and the printing industry are extending into a broad range of applications, concomitantly with the varietization of packaging containers. For example, in the field of containers such as the cups used for those food products generally called as “cup noodles,” including ramen products, udon and soba noodles, which become edible several minutes after an appropriate amount of boiling water is poured into the container, polystyrene cups have been mainly used from the viewpoint of cost and heat insulating properties.
However, in the case of foamed polystyrene cups, printing on the cups must be performed individually after forming of the cup. Accordingly, the printing method is limited to methods of using a curved surface printing machine or a stamp printing machine, and there has been a problem that the printing speed and the print quality are poor. Furthermore, owing to the reasons such as that the polystyrene price is increasing concomitantly with a recent increase in the price of petroleum and that under the implementation of the Containers and Packaging Recycling Law, the food product manufacturers are responsible for the recycling fee, transfer from polystyrene cups to paper cups is being promoted in recent years.
As one of the representative structures of paper containers, there is known a double cup having a heat insulating air layer which is formed by bonding paper and paper with furrows therebetween. However, the double cup has problems of high weight and high price. Therefore, heat insulating, foamed paper containers are attracting more attention these years.
A heat insulating foamed paper container is generally produced using a paper container material having a thermoplastic synthetic resin film that is foamed by a heat treatment carried out upon production of the container and forms a heat insulating layer. More specifically, the paper container material has a structure in which, for example, a high-melting point polyethylene film having a melting point of about 130° C. to 135° C. is laminated on one surface of a base paper (inner side of container), and a low-melting point polyethylene film having a melting point of about 105° C. to 110° C. (hereinafter, referred to as “low-Mp film”) is laminated on the other surface of the base paper (outer side of container), and formed on the surface of the low-Mp film is a printed layer containing print patterns such as decorative patterns, the company name and a barcode.
In the production process for the heat insulating foamed paper container, a paper container material having a printed layer formed in advance is punched out into a predetermined shape, this is used as a body member of cup to form a cup, and then the cup is subjected to a heat treatment near the melting point of the low-Mp film, thereby foaming the low-Mp film. Foaming occurs when the moisture contained in the base paper evaporates at the time of the heat treatment and is pushed out to the side of the low-Mp film which has been softened, and thereby the low-Mp film swells toward the outer side. The low-Mp film thus foamed functions as a heat insulating layer and imparts heat insulating properties to the paper container. Such a heat insulating foamed paper container is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 57-110439 (Patent Document 1).
The formation of a printed layer on the surface of the low-Mp film of the material for heat insulating foamed paper containers is usually carried out according to a surface printing method based on gravure printing. In the field of surface printing based on gravure printing, ink is applied on the surface of objects to be printed, such as various base papers and plastic packaging containers. Therefore, inks for gravure printing are requested to have printing suitability for objects to be printed, as well as adhesiveness to the base papers and plastic materials, and various resistances such as the resistance to blocking to prevent adhesion between printed matters after printing, and the abrasion resistance and heat resistance required upon the formation of container.
Furthermore, an ink for gravure printing usually contains an aromatic solvent such as toluene as a main solvent, so as to achieve a balance between the solubility of binder resins and the dryability. However, recently, the tendency toward using a printing ink that does not contain an aromatic solvent is growing stronger, in order to improve the environment of the printing work site.
Moreover, when the product value as a heat insulating foamed paper container is considered, the ink for gravure printing for forming a printed layer of the container is desirably such that the ink film does not impede but promotes foaming of the low-Mp film that constitutes the heat insulating layer and can thereby provide a container surface having less surface unevenness, and that the ink film has less occurrence of cracks and can provide a smooth printed surface.
As the ink for gravure printing for forming printed layers of heat insulating foamed paper containers, there has been conventionally known an ink containing a polyamide resin and a cellulose derivative as binder components. However, in the case of an ink containing a polyamide resin as a main component of the binder components, when the ink film is exposed to heat or light over a long time, the low molecular weight components such as oils and fats contained in the polyamide resin undergo oxidative degradation into acetaldehydes, thereby causing a problem of generating a fat-based odor. Furthermore, since there is a possibility that the content in the container may cause a change in taste due to the generated fat-based odor, this poses a serious problem. In addition, the printing ink mentioned above markedly suppresses the foaming of the low-Mp film, and thus there occur differences in the thickness of the low-Mp film after foaming, between a printed area and an unprinted area of the printed layer, or between a printed area and an overprinted area where further printing has been applied on an existing printed area. Therefore, the printed areas or overprinted areas on the container surface become concave areas, and obvious level differences occur. Thus, a smooth sense of touch is not obtained, and there are problems such as impaired merchantability or unreadable barcodes.
For the purpose of improving the surface unevenness of the printed layer in conventional heat insulating foamed paper containers, JP-A No. 11-189279 (Patent Document 2) proposes a technology of using, on the surface of a low-Mp film, an ink that can be synchronized with the foaming of the low-Mp film without inhibiting the foaming. Furthermore, JP-A No. 2001-270571 (see Patent Document 3) discloses a specific composition of the synchronous ink. The component blend of the white ink disclosed in Patent Document 3 includes 30 to 35% of an inorganic pigment, 15 to 25% of a urethane-based resin, 0 to 5% of a vinyl chloride acetate resin, 15 to 20% of a ketone-based solvent, 5 to 10% of an acetic acid ester-based solvent, 0 to 10% of an alcohol-based solvent, 0 to 10% of a toluol-based solvent, and 1 to 5% of a polyethylene wax and other auxiliary agents. The component blend of a color ink includes 5 to 10% of an organic pigment, 5 to 10% of a urethane-based resin, 0 to 5% of a vinyl chloride acetate-based resin, 40 to 50% of a ketone-based solvent, 15 to 25% of an acetic acid ester-based solvent, 5 to 10% of an alcohol-based solvent, 0 to 10% of a toluol-based solvent, and 1 to 5% of a polyethylene wax and other auxiliary agents. The synchronous ink is disclosed as an ink suitable for forming an undercoat layer.    Patent Document 1: JP-A No. S57-110439    Patent Document 2: JP-A No. H11-189279    Patent Document 3: JP-A No. 2001-270571